Toll-like receptor (TLR)

Structure and localisation

TLRs are type I transmembrane proteins with three characteristic regions: an extracellular leucine-rich repeat (LRR) domain that binds ligands, a single transmembrane helix, and a cytoplasmic Toll/Interleukin-1 receptor (TIR) signalling domain. Different family members are distributed either on the plasma membrane (often sensing bacterial cell-surface components) or on endosomal membranes (commonly sensing nucleic acids). This compartmentalisation influences which microbial molecules each TLR recognises and how the receptor is activated.

Ligands and downstream signalling

Each TLR recognises particular molecular signatures, called pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs). Examples include bacterial lipopolysaccharide, flagellin, lipopeptides and microbial nucleic acids. Ligand binding induces receptor dimerisation and recruitment of adapter proteins such as MyD88 or TRIF, which lead to activation of transcription factors including NF-κB and interferon regulatory factors (IRFs). The result is production of cytokines, chemokines and type I interferons that coordinate inflammation and anti-microbial defence.

Biological roles and medical relevance

TLRs provide a rapid first line of defence at epithelial barriers such as the skin and gut and in innate immune cells like macrophages and dendritic cells. By promoting dendritic cell maturation and expression of co-stimulatory molecules, TLR signalling is a principal mechanism linking innate detection to adaptive responses. Dysregulated TLR activity contributes to inflammatory diseases, septic shock, autoimmunity and can influence tumour microenvironments. For these reasons, TLR agonists and antagonists are under investigation as vaccine adjuvants, immunotherapies and anti-inflammatory agents.

History and discovery

The name "Toll" originates from genetic studies in the fruit fly, where the receptor was first characterised for roles in development and antifungal defence. Subsequent work identified related receptors in vertebrates and clarified their immunological functions, transforming the understanding of innate immunity. This discovery established TLRs as central sensors that trigger complex gene programmes in response to infection.

Further notes and resources

• For basic descriptions and curated summaries see general overview and introductory resources.
• Structural and localisation details are discussed in specialized reviews: structure, cellular distribution.
• Signalling pathways and adapters: MyD88-dependent signalling, TRIF-dependent signalling.
• Clinical and translational aspects, including therapeutic modulation, are covered at clinical perspectives and therapeutic research.
• For immunological context and the role of dendritic cells see dendritic cell interactions.